Recording materials using electron donating dye precursors and electron accepting compounds are well known as pressure-sensitive recording papers, heat-sensitive recording papers, light-sensitive and pressure-sensitive recording papers, and electric heat-sensitive recording papers etc. The details of these recording materials are disclosed, for example, in British Patent 2,140,449, U.S. Pat. No. 4,480,052 and 4,436,920, JP-B-60-23922, JP-A-57-179836, JP-A-60-123556 and JP-A-60-123557 (The terms "JP-A" and "JP-B" as used herein mean "unexamined published Japanese patent application" and "examined Japanese patent publication", respectively).
These recording materials are required (1) to have an image having sufficient color density with color forming sensitivity, (2) to have a good dot reproducibility and excellent in granularity and gradation properties, (3) not to fog, (4) to have an image having sufficient fastness after color formation, (5) to form a hue suited for copying machines, (6) to have a high S/N ratio, and (7) to have a colored image sufficiently resistant to chemicals, and the like. However, none of the conventional recording materials has completely fulfilled these requirements.
Various recording systems have been investigated with a view to resolving these requirements, and remarkable achievements have been made with heat-sensitive recording materials. Heat-sensitive recording materials having a higher sensitivity, i.e., a sufficient color density obtained with a lower printing energy, are required in order to reduce the printing energy which has accompanied miniaturization of the equipment and to shorten the sending times. Further since heat-sensitive papers are used for the output of high image quality comparable to that silver salt photography, a high image quality heat-sensitive paper which has improved dot reproducibility of the printing head is also required.
To meet these demands for higher sensitivity and higher image quality, methods of increasing the heat responsibility of the heat-sensitive color forming layer materials and of improving the smoothness of the recording paper surface in order to improve the contact between the thermal head and the recording paper have been investigated. However, there are limits with these methods and really satisfactory materials have not yet been obtained.